1. Field of the Invention
The present invention relates to an image processing device, an imaging device, a computer-readable recording medium recording an image processing program, and an image processing method, and particularly relates to an image processing device, an imaging device, a computer-readable recording medium recording an image processing program, and an image processing method for processing a fluorescence image based on fluorescence from a subject.
2. Description of the Related Art
Conventionally, endoscope apparatuses have been used in the medical field to observe inside of organs of a subject. The endoscope apparatuses generally capture a white light image of a body tissue by inserting an elongated flexible insertion portion into a body cavity of a subject such as a patient, applying white light to the body tissue in the body cavity via the inserted flexible insertion portion, and receiving reflected light by an imaging unit provided inside the flexible insertion portion. The white light image of the body tissue is displayed on a display unit of the endoscope apparatus. A user such as a doctor observes inside of the body cavity of the subject by using the white light image of the body tissue displayed on the display unit of the endoscope apparatus.
Meanwhile, in the field of endoscopes, there has been proposed in recent years an endoscope apparatus that is able to perform fluorescence observation of an observed region such as a body tissue inside a body cavity. Such a fluorescence-observation endoscope apparatus generally applies excitation light to the body tissue inside the body cavity via an inserted flexible insertion portion, and captures a fluorescence image of the observed region based on fluorescence generated from the body tissue.
When the observed region is a region in a luminal organ such as an esophagus or an intestine inside the subject, it is likely that an imaging distance between the observed region and the imaging unit of the endoscope apparatus is not constant. Therefore, when the imaging unit of the endoscope apparatus captures a fluorescence image of the observed region, light-dark contrast of fluorescence is caused because of variation in the imaging distance between the imaging unit and the observed region as an imaging object. The light-dark contrast of fluorescence makes it difficult to distinguish between a normal body tissue and an abnormal tissue during the fluorescence observation of the observed region.
To solve the problems mentioned above, the fluorescence-observation endoscope apparatus generates a standardization image based on light reflected from the observed region irradiated with light such as white light, performs a standardization process of dividing the fluorescence image of the observed region by the generated standardized image, and corrects lightness and darkness of the fluorescence caused by the above-described variation in the imaging distance between the imaging unit and the observed region. Then, the fluorescence-observation endoscope apparatus displays the fluorescence image, in which luminance values are standardized through the standardization process, on the display unit. A user such as a doctor observes the fluorescence image of the observed region displayed on the display unit of the fluorescence-observation endoscope apparatus, and makes a diagnosis of presence or absence of an abnormal tissue such as an affected area in the observed region inside the subject by the fluorescence observation.
When the observed region as an object of the above-described fluorescence observation contains a contrast region that forms contrast between light and dark (e.g., a blood vessel in a superficial layer or a deep blood vessel in a mucous membrane of the body tissue), a standardization image generated based on reflected light from the observed region contains, corresponding to the contrast region, a pixel portion having luminance largely different from that of a neighboring pixel, i.e., an edge. When the fluorescence image of the observed region is divided by the standardization image containing the edge, a luminance value of each pixel corresponding to the edge in the standardization image affects the standardization process, and an S/N ratio of the fluorescence image of the observed region may be decreased.
To prevent the decrease in the S/N ratio of the fluorescence image caused by the edge in the standardization image, there has been proposed an apparatus that performs a low-pass filter process on the standardization image based on the reflected light from the observed region, and divides the fluorescence image of the observed region by the standardization image subjected to the low-pass filter process (see, for example, Japanese Laid-open Patent Publication No. 2002-336187). Furthermore, there has been proposed an apparatus that captures a near-infrared light image of the observed region by applying near-infrared light to the observed region, and divides an image signal of the fluorescence image of the observed region by an image signal of the captured near-infrared light image (see, for example, Japanese Laid-open Patent Publication No. 09-294706).